New radar retrieval technique may give forecasters a clearer picture of how much rain could fall during a storm

October 11, 2016

Example of raindrop and air motion data collected from collocated vertically pointing radars during the MC3E field campaign.

Example of raindrop and air motion data collected from collocated vertically pointing radars during the MC3E field campaign.

Cloud systems are a churning, complex mixture of interactions—the right combination of
ingredients can quickly lead to dangerous conditions, such as flash floods. More accurate
observations of these factors are important for improving forecasts and warnings. Christopher
Williams, a CIRES researcher at ESRL's Physical
Sciences Division, and colleagues from Colorado State University have developed a new radar
retrieval technique that provides better estimates of the number and size of raindrops along with the
strength of the surrounding updraft (or downdraft), which may give forecasters a clearer picture of how
much rain could fall during a storm—and how fast the raindrops fall.

"By pointing two radars at the same volume of raindrops, this new retrieval technique
simultaneously estimates raindrop populations and cloud dynamics," said Williams.

Vertically-pointing radars can observe raindrop Doppler motion—a combination of falling
raindrops embedded in the moving atmosphere. Strong updrafts cause raindrops to ‘fall’ upwards,
lifting them to colder heights, which promotes raindrop freezing and the formation of dangerous
hail. When using only one vertically pointing radar, it is difficult to isolate raindrop motion
from air motion. Interestingly, the 'apparent' size of raindrops is dependent on the radar
operating frequency because the radar measured raindrop motion decreases as the radar frequency
increases. Consequently, raindrops appear to fall slower when observed with higher frequency
radars while the air motion is independent of radar frequency. Using this information, the
researchers analyzed raindrop motion measured by two different frequency radars operating
side-by-side during the
Midlatitude Continental Convective Clouds Experiment
to estimate raindrop population and air motion. Their findings
are published in the October issue of IEEE Transactions on Geoscience and Remote Sensing.

"Improving weather forecast models is a cascading sequence," said Williams. "Improved radar
rainfall retrievals will lead to improved understanding of cloud physics, which will lead to
improved model parameters, which will eventually improve weather forecast models."